1. Field of the Invention
The present invention relates to a control apparatus of an electric power steering system for applying a steering assist force by means of a motor to a steering system of a car or a vehicle, and particularly to a control apparatus of an electric power steering system with improved control characteristics and safety having two control parts (CPUs or MCUs: Micro Controller Units), a main and a sub, and carrying out motor control with one and monitoring for abnormality of the drive system with the other.
2. Description of the Prior Art
An electric power steering system for urging with an assisting load a steering system of a car or a vehicle using rotational force of a motor applies as an assisting load to a steering shaft or a rack shaft a driving force of the motor by means of a transmission mechanism such as gears or a belt by way of a speed-changer. This electric power steering system of prior art, to generate an assist torque (steering assist torque) correctly, performs feedback control of a motor current. The feedback control regulates a motor impressed voltage so that the difference between a current command value and a motor current detected value decreases, and regulation of the motor impressed voltage is generally carried out by regulation of a duty ratio of PWM (Pulse Width Modulation) control.
Here, showing in FIG. 1 and explaining a common construction of an electric power steering system, a shaft 2 of a steering wheel 1 is connected to a tie rod 6 of steering vehicle wheels via universal joints 4a and 4b and a rack and pinion mechanism 5. A torque sensor 10 for detecting a steering torque in the steering wheel 1 is provided on the shaft 2, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the shaft 2 by way of a clutch 21 and reduction gears 3. To a control unit 30 for controlling the power steering system power is supplied from a battery 14 via an ignition key 11 and a relay 13, and the control unit 30 performs computation of a steering assist command value I of an assist command on the basis of a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12 and controls a current supplied to the motor 20 on the basis of the computed steering assist command value I. The clutch 21 is ON/OFF-controlled by the control unit 30, and in a normal operating state is ON (engaged). When it has been determined by the control unit 30 that the power steering system has failed, and when due to the ignition key 11 or the relay 13 the power supply (voltage Vb) from the battery 14 is OFF, the clutch 21 is made OFF (disengaged).
The control unit 30 consists mainly of a CPU, and general functions executed by a program inside the CPU are shown in FIG. 2. For example a phase compensator 31 does not denote a phase compensator consisting of independent hardware, but rather denotes a phase compensation function executed by the CPU. Explaining the functions and operation of the control unit 30, the steering torque T detected by the torque sensor 10 and inputted is phase-compensated by the phase compensator 31 to raise the stability of the steering system, and a phase-compensated steering torque TA is inputted to a steering assist command value computing element 32. The vehicle speed V detected by the vehicle speed sensor 12 is also inputted to the steering assist command value computing element 32. The steering assist command value computing element 32 determines on the basis of the inputted steering torque TA and vehicle speed V a steering assist command value I, which is a control target value of the current supplied to the motor 20. The steering assist command value I is inputted to a subtractor 30A and is also inputted to a differential compensator 34 of a feedback system, the differential (I-i) of the subtractor 30A is inputted to a proportional computing element 35, and the proportional output thereof is inputted to an adder 30B and is also inputted to an integral computing element 36 for improving the characteristics of the feedback system. The outputs of the differential compensator 34 and the integral computing element 36 are also additively inputted to the adder 30B, and a current command value E, which is the addition result in the adder 30B, is inputted to a motor drive circuit 37 as a motor drive signal. The motor current value i of the motor 20 is detected by a motor current detecting circuit 38, and the motor current value i is fed back by being inputted to the subtractor 30A.
As described above, in the past a current command value has been computed on the basis of a torque signal (the steering torque T) and the current detected value i with one CPU, and the motor drive-controlled on the basis of that current command value. In this case, with respect to the determined motor current drive direction, by hard logic or another CPU a steering torque direction signal has been generated on the basis of the torque signal, and the motor has been driven only when this steering torque direction signal and the motor drive direction match. When detecting an abnormality of the motor drive system, the time from detecting to confirming the abnormality has been fixed and not correct always.
In the past the one CPU has been provided with an externally added WDT (Watch Dog Timer), and racing of the CPU has been monitored by clear pulses from the CPU being inputted to the WDT. If no clear pulse is inputted within a predetermined time, a reset signal has been outputted to the CPU from the WDT and the CPU has thereby been restarted. Also, in a system using two CPUs they have each outputted a pulse of a predetermined period to the other and monitored racing of the other CPU by mutually monitoring the period of the pulse.
In an electric power steering system of the kind described above, as the output of the motor increases the inertia of the motor increases, and to secure steering characteristics the need to compensate for the inertia of the motor has emerged. As a result of the control compensating for inertia, the direction according to the torque signal which has been carried out in the past and the direction in which the motor is actually driven have come to not match. Consequently, the problem has arisen that notwithstanding that the control containing the inertia compensation function is operating normally, due to another control part outputting a torque direction signal based on the torque signal, the motor drive has temporarily been stopped. That is, direction interlock based on the direction of the steering torque, which has been carried out in the past, is becoming unsuited to actual systems.
In the past, when detecting abnormality of the motor drive system, the time from detecting to confirming the abnormality has been fixed. However, when the difference between the current command value and the motor current detected value is obtained to detect abnormality, when there Is a large difference it can be considered that there is clearly abnormality, and it is necessary to stop the system without delay. And, when the difference is small, because a mis-detection caused by the influence of noise or the like is conceivable, it is necessary to take time to confirmation.
When racing of the CPU is monitored using an externally added WDT as in the past, when racing arises in the CPU due to the program not operating normally, it is restarted by a reset signal from the WDT. However, in the case of a failure of a kind such that the program causes racing again at a similar place, an outputting state and a stopped state of the motor occur alternately, and have created a dangerous state for the driver.
On the other hand, in a system having two control parts, when one control part performs control computation and the other control part monitors that, to check that the monitoring control part is operating normally it is necessary for the two control parts to monitor racing of the CPU mutually.
When in a system using two CPUs they send and receive pulse signals to each other to monitor racing of the CPUs, there have been cases wherein the program itself does not operate normally and only continues to output the monitoring pulse so that racing cannot be detected, or due to electromagnetic waves or noise a pulse signal being outputted normally is measured erroneously and a mis-detection is made.
The present invention was made on the basis of the kind of situation described above, and an object of the present invention is, particularly when controlling a motor with large inertia, to provide a control apparatus of an electric power steering system with improved control performance and safety having two digital control parts (CPUs or MCUs), a main and a sub, and carrying out control of power steering with one and mutually monitoring with the other control part.
The present invention relates to a control apparatus of an electric power steering system for, on the basis of a current command value computed from a current detected value of a motor and a steering assist command value computed on the basis of a steering torque arising in a steering shaft, controlling the motor, which applies a steering assist force to a steering mechanism, and the above-mentioned object of the present invention is achieved by there being provided a first control part and a second control part each inputting at least a torque signal, a vehicle speed signal, the motor current detected value and a motor terminal voltage, and the motor being controlled with the first control part and the second control part.
Also, the above-mentioned object of the present invention can be achieved by it having and controlling the motor with a first control part and a second control part each inputting at least a torque signal, a vehicle speed signal, the motor current detected value and a motor terminal voltage; both of the control parts having a function of estimating the motor angular speed; and the first control part and the second control part being connected by serial communication and each monitoring whether the other control part is racing by transmitting and receiving predetermined communication commands between themselves, and performing self-monitoring with WDTs built in to the MCUs.